1. Field of the Invention
This invention relates to acoustic range finding systems of the type in which an electro-acoustic transducer transmits a pulse or shot of acoustic energy towards a surface whose distance is to be measured, and subsequent signals received from the transducer are monitored to determine the temporal location of an echo from that surface.
2. Review of the Art
Different transducer characteristics and operating parameters are required for use in different circumstances. In general, the best performance at short ranges is obtained using transducers operating at relatively high frequency, typically of the order of 50 kHz for ranges up to 20 meters or so, whilst at longer ranges, transducers operating at lower frequencies provide better performance, with frequencies of the order of 12 kHz being suitable for very long ranges of 50 meters or more, and intermediate frequencies for intermediate distances. Broadly speaking, the higher frequencies provide more sharply defined echoes and better resolution, but are subject to more rapid attenuation with distance particularly under adverse conditions such as dusty environments, whilst low frequencies are less subject to attenuation but provide more diffuse echoes and lower resolution. The reflectivity and shape of surfaces whose position is to be determined also varies with frequency, and thus the identity of the substance whose surface is to be measured, and its angle of repose, may influence the choice of transducer frequency. For example, granular material with a sloping surface tends to reflect low frequencies against a wall of the vessel thus producing a weak direct echo, and strong indirect echoes reflect one or more times from the wall of the vessel. This characteristic is less marked with higher frequencies, but in deep vessels it is not practicable to use as high a frequency as would be desirable to mitigate this problem. It is common practice to utilize longer pulse widths with lower frequencies, both to allow the transmission of more sonic energy, and to allow for the slower response time of low frequency transducers. Since the received signal is usually very small compared with the transmitted signal, and is subject to high levels of noise, both the transmitter and receiver are turned close to the resonant frequency of the transducer so as to optimize the signal-to-noise ratio of the system. The transmission frequency is sometimes slightly offset from the actual resonant frequency of the transducer for various reasons; thus it is known to tune the transmitter for optimum echo amplitude, which may occur at a slight offset from the nominal resonant frequency, and also to shift or sweep the transmitter frequency over a small range so as to avoid nulls in the echo response due to interference effects within the environment being monitored. Such transmitter frequency changes are fairly small, and can be accommodated within the bandwidth of the receiver.
U.S. Pat. No. 4,199,246 (Muggli), issued Apr. 22, 1980, describes an ultrasonic ranging system in which the transmitter is driven by a voltage controlled oscillator, such that the frequency transmitted by the transducer is changed in a predetermined manner over a substantial range during the course of the transmitted pulse. The bandwidth of the receiver is varied again according to a preset pattern, during a subsequent period so that the receiver bandwidth is narrowed with the passage of time following the pulse, the passband of the receiver being centered upon the lowest frequency transmitted. By configuring the transmitted pulse so that a short initial portion is transmitted at a relatively high frequency, which is then reduced in one or more steps to a relatively low frequency, and configuring the receiver so that its initial bandwidth is wide enough to pass the highest as well as the lowest frequency, short range echoes of the high frequency pulse components may be detected, but at longer ranges, reception of the low frequency component and exclusion of noise is optimized, by decreasing the bandwidth and thus improving the quality factor (Q) of the receiver.
The Muggli system is subject to two constraints which limit its applicability. The transducer itself must be capable of operation over a wide range of frequencies, and the noise immunity of the system at short ranges is very poor because of the wide bandwidth of the receiver at those frequencies. Neither of these limitations need be serious in the camera control applications for which the Muggli system is clearly primarily designed, involving as they do low power transducers, comparatively short ranges, and environments which are comparatively quiet at the frequencies of interest; they are however highly significant in typical industrial applications for which suitable transducers operating over a wide frequency range are not generally available. Instead it has been necessary to select a suitable transducer, and to provide a transmitter/receiver system whose frequency characteristics and output voltage are matched to the transducer.
In order to overcome this problem, it is known to provide transducers with an integral matched transceiver unit and a preprocessor for received signals which converts received echo information into data of standardized format which is essentially independent of the transducer type. This standardized data can then be transmitted for further processing at a remote point, in a manner independent of the transducer characteristics. Such an arrangement is disclosed in U.S. Pat. No. 4,700,569 (Michalski). Whilst such an arrangement simplifies the remote processing unit for a transducer, and enables it to handle data from different types of transducers, this is at the expense of the addition of complex circuitry to each transducer.